EP 0 477 400 discloses a system for an adaptive, mechanical tolerance compensation, effective in the lift direction, for a path transformer of a piezoelectric actuator for a fuel injector. The actuator lift is transmitted via a hydraulic chamber in this case. The hydraulic chamber has a defined leakage with a defined leakage rate. The lift of the actuator is initiated into the hydraulic chamber via a transmitter master piston and transmitted to an element to be operated via a receiver slave piston. This element, for example, is a valve needle of a fuel injector.
EP 0 477 400 discloses a path transformer for a piezoelectric actuator in which the actuator transmits a lifting force to a transmitter cylinder which is sealed by a cylinder support. Guided in this transmitter cylinder is a receiver piston which likewise seals the transmitter cylinder and thereby forms the hydraulic chamber. A spring which pushes the transmitter cylinder and the receiver piston apart is positioned in the hydraulic chamber. The receiver piston mechanically transmits a lifting movement to a valve needle, for instance. When the actuator transmits a lifting movement to the transmitter cylinder, this lifting movement is transmitted to the receiver piston by the pressure of an hydraulic fluid in the hydraulic chamber since the hydraulic fluid in the hydraulic chamber is not compressible and only a very small portion of the hydraulic fluid is able to escape through the annular gap during the short duration of a lift. In the rest phase, when the actuator does not exert a pressure force on the transmitter cylinder, the spring presses the receiver piston out of the cylinder and, due to the generated vacuum pressure, the hydraulic fluid enters the hydraulic chamber via the annular gap and refills it. In this way, the path transformer automatically adapts to longitudinal deformations and pressure-related extensions of a fuel injector.
This known art is disadvantageous in that the hydraulic chamber can only be filled slowly. Long injection times occur especially in a cold start at low pressure, so that more hydraulic fluid escapes via the annular gap and must subsequently be refilled in a shorter period of time at low pressure. If this is not done, the fuel injector loses lift in each injection until it is entirely unable to function.
It is also disadvantageous that the hydraulic fluid can evaporate if insufficient pressure prevails in the hydraulic chamber. However, gas is compressible and generates an appropriately high pressure only after a considerable reduction in volume.
This poses a particular danger when shutting off a hot internal combustion engine which uses a fuel injector for gasoline and in which the gasoline is simultaneously used as the hydraulic fluid. A fuel injection system then loses its pressure, and the gasoline evaporates particularly easily. In a new effort to start the internal combustion engine, this may result in the lifting movement of the actuator not being transmitted to the needle since the following flow of cool fuel does not reach the hydraulic chamber soon enough.